Yogurt
Introduction As a Persian tradition states, “Abraham owed his fecundity and longevity to the regular ingestion of yogurt". This divine delight has been around for countless generations and some would even suggest since the early beginnings of life on Earth. But what exactly is yogurt? And why does it occupy so much real estate in modern-day grocery stores? Some would argue that yogurt is a harmonious amalgamation of protein, calcium, folate, vitamin B12, and phosphorous (to name a few), which synergistically lend themselves to an explosion of taste that yogurt lovers have come to expect from this creamy, smooth concoction (Yale-New Haven Nutrition Advisor, 2005). Simple science points to basic metabolism as the foundation of explaining this ancient marvel. Yogurt is made from the fermentation of lactose in cow’s milk through two well-known bacterial species – Streptococcus thermophiles and Lactobacillus delbrueckii ''subsp ''bulgaricus. ''Fermentation is a metabolic process that converts a carbohydrate sugar into an acid, gas, or alcohol product – in the case of yogurt, the disaccharide lactose is converted into lactic acid. Moreover, the lactic acid, in turn, increases the acidity of the milk culture and consequently gives rise to its eventual gel-like structure and tart taste. Ultimately, it is invaluable to fully understand all the biochemical processes involved with fermentation as it applies to the production of yogurt. With more investigation, more opportunities may unfold in terms of not only improving the quality of yogurt produced but also sensual characteristics such as taste and texture. Considering the delightful taste, nutrients and potential health benefits, it’s no wonder the global yogurt market is expected to exceed sixty-seven billion dollars by 2015 (Global Industry Analysts Inc, 2012). It has become a modern-day staple in most kitchens around the world and we can only hope this trend continues. Yogurt Production '''Large Scale Production' Milk Standarization Milk solid contents are often adjusted by mixing milk with skim milk, cream or milk powders (Lee, 2013). Milk Powders, including non-fat dry milk, whey protein concentrates, or milk protein concentrate, can all be blended with milk (Lee, 2013). The milk solids content ranges from 9% (skim milk) to 20% (concentrate) (Lee, 2013). Certain yogurt properties including texture, consistency, appearance, viscosity and the prevention of whey separation can be maintained by the use of stabilizers, such as pectin or gelatin (Lee, 2013). Homogenization To maintain the fat content within yogurt, milk base is homogenized, preventing fat separation during the process of fermentation (Lee, 2013). Homogenization can improve other properties including whey separation, yogurt consistency and appearance (Lee, 2013). Under the temperature range of 55-65°C, the standard pressure settings include: • First stage pressure: 10-20 MPa • Second stage pressure: 5 MPa • Ultra- high: 200-300 MPa · Pasteurization The milk heat treatment influences the physical properties and microstructure of yogurt (Lee, 2013). It can also destroy unwanted microbes and remove dissolved oxygen, encouraging anaerobic fermentation and improving yogurt quality (Lee, 2013). After pasteurization, bacterial cultures can be added (Lee, 2013). A few common settings for heat treatment include: • 85°C for 30 min • 90-95°C for 5min • High temperature short time (100°C - 130°C for 4 - 16 s) • Ultra-heat temperature (140°C for 4 - 16 s) Fermentation After pasteurization, milk is cooled to the incubation temperature of 40°C to 45°C for the growth of thermophilic acid bacteria (Lee, 2013). The temperature decrease also allowes for the conversion of lactose to lactic acid, decreasing the pH from 6.7 to ≤ 4.6 (Lee, 2013). The decrease in pH causes gelation which is one of the key steps in forming yogurt (Lee, 2013). The two thermophilic bacteria most commonly used are Streptococcus thermophilus and Lactobacillus delbrueckii bulgaricus. Cooling The cooling process continues until the mixture reaches 20°C (Lee, 2013). At this temperature, fruit or flavouring ingredients can be added (Lee, 2013). At less than 10°C, further acidification continues (Lee, 2013). After this initial cooling process, the yogurt must undergo a different second cooling step depending on whether it is stirred or set (Lee, 2013). Set yogurt enters the cold storage directly, reaching 10°C (Lee, 2013). Stirred yogurt has to be cooled by agitation, and then placed in cold storage (Lee, 2013). Small Scale Production – Making it at Home Home-made yogurts give rise to certain advantages of being simple, cheap, and organic with no additives/sweeteners (Diane, 2009). Small scale production requires 4 cups of milk and 2 tablespoons of plain yogurt with active cultures (Diane, 2009). Relevance of Biochemical Pathways in Yogurt Production The significance of certain biochemical pathways depends on the type of organism and the environment. The two main bacteria used in the yogurt manufacturing process are Lactobacillus bulgaricus and Streptococcus thermophilus. These are both facultatively anaerobic organisms, meaning that they can survive in both aerobic and anaerobic environments and switch between fermentation and oxidative phosphorylation (Horiuchi, 2009). In the production of yogurt, the main source of sugar used is the lactose present in milk (Lee, 2010). Lactose is broken down into glucose and galactose, which can then feed into glycolysis. Glucose enters glycolysis directly. Galactose, on the other hand, is converted into glucose-1-phosphate which is then converted to glucose-6-phosphate, an intermediate in glycolysis. The glucose is initially converted to pyruvate through glycolysis (MacDonald, 2013). Glycolysis then produces pyruvate. Under oxygen-rich conditions, the pyruvate is then converted into acetyl-CoA, which can enter the tricarboxylic acid cycle (TCA cycle). NADH and FADH2 produced in glycolysis and the TCA cycle drop off electrons at the electron transport chain, which are used to create a proton gradient. This proton gradient is then taken advantage of to produce large amounts of ATP. Under oxygen-deficient conditions, or when there is a need for an immediate source of ATP, the pyruvate produced in glycolysis acts as the final electron acceptor through fermentation. In lactic acid bacteria, the fermentation process produces lactic acid (MacDonald, 2013) . Generally, oxidative phosphorylation is carried out in the presence of oxygen and fermentation is used mainly in oxygen –deficient situations or when there is a rapid need for ATP. However, since L. bulgaricus and S. thermophilus are facultative anaerobes, they are able to use fermentation efficiently in the presence of oxygen (Horiuchi, 2009). This suggests that glycolysis and lactic acid fermentation are the most significant biochemical pathways used in yogurt production. Effect of Lactate Concentration on Yogurt Properties As the concentration of lactic acid produced increases, the solution becomes more acidic, decreasing the pH from 6.7 to ~4.6. This is essential for the physical properties of yogurt. Milk that has undergone high heat treatment attains a gelatin-like texture at a pH of around 5.2-5.42. Thus, lactic acid is important in the smoothness of the yogurt. Acidity due to increased lactate concentration is also associated with sourness and a bitter aftertaste (Ott, 2000). pH and Temperature Optima Optimal growth conditions were found to be pH 6.5 and 40°C for S. thermophilus ''and pH 5.8 and 44°C for ''L. bulgaricus. The pH and temperature optima of the acidification characteristics differ from those of growth: they are consistently higher by 0.1 to 0.2 pH units and by about 5 ° C, except in the case of L. bulgaricus, for which the optimal temperature is in all cases equal to 44 ° C (Beal, 1989). Facultative vs. Obligate Anaerobes S. thermophilus is a facultative anaerobic organism. A facultative anerobe produces ATP by aerobic respiration in the presence of oxygen but is also capable of switching to fermentation. L. bulgaricus is also a facultative anaerobic organism. Obligate anaerobes die in the presence of oxygen so they can only undergo fermentation and cannot produce ATP by aerobic respiration (Beal, 1989) Symbiosis Between S. thermophilus and L. bulgaricus Acetaldehyde production during the first few hours of incubation can be used as an indicator of yogurt ripening (Hamdan et al., 1971). It has been experimentally determined that using S. thermophilus and L. 'bulgaricus in combination yields higher acetaldehyde production as compared to using only one species or the other. The attached figure illustrates significantly more acetaldehyde in a 1:1 ratio of the two species in a starter culture relative to production by one species independently. Coutin, P. and Rul, F. (2004) suggest a mechanism for the synergistic acidification of milk in combined cultures. They find that L. bulgaricus benefits from pyruvic acid, formic acid, and CO2 produced by S. thermophilus. S. thermophilus, being more weakly proteolytic than L. bulgaricus, benefits from the specie’s production of peptides and amino acids. Four essential amino acids of S. thermophilus are Met, His, Pro, and Glut, out of which three are produced by L. bulgaricus as free amino acids. The symbiosis appears more beneficial for S. thermophilus who experiences more population growth than L. bulgaricus with respect to pure cultures. This may be due to relatively higher nutritional requirements of L. bulgaricus. The two cultures compete, and S. thermophilus has a competitive advantage resulting from faster growth in milk. Sensory Evaluation Sensory evaluation is a critical facet of the entire production process as it provides a concrete mechanism through which suppliers can ensure consumers are provided the texture, taste and overall quality of yogurt that they have long been accustomed. Both theoretical and experimental data is required for the thorough understanding of a topic. Similarly, in the field of yogurt production, theoretical statistics, such as chemical composition is not adequate to develop a holistic picture of a product. Experimental data in the nature of sensory evaluation is required, and if performed accurately, would provide the greatest insight to predict a products success (Sidel, 1993). Furthermore, because there are no objective standards of testing for parameters such as consistency, sensory evaluation is one of few methods of product evaluation that is available to researchers. One of the major factors to ensure accurate evaluation is that the tasters need to be from a large pool of regular yogurt consumers, rather than a panel of expert tasters (Stone, 2004). This will avoid bypassing any biases by the experts, which are present in the actual consumers. Furthermore the opinion from the consumers is the most important, because they are the people who are going to ultimately pay money and buy the product. Low Fat Dairy Products' Effect on Type 2 Diabetes The consumption of dairy products has been proven to reduce obesity and promote a healthy lifestyle. Therefore these products have become an interest in the field of research, especially the effects of dairy products on type 2 diabetes. The prevalence of type 2 diabetes has been increasing at a rapid rate. It has been predicted that in 2030 more than 330 million individuals will have type 2 diabetes (Wild S., 2004). Recent studies have shown that people taking low-fat dairy products have an 18% lower risk of developing type 2 diabetes (Tong X., 2011). In another study, long-term consumption of 4 daily servings of low-fat milk or yogurt has shown to reduce fasting plasma insulin and also increase insulin resistance by 9% and 11% respectively (Rideout T., 2013). It has not been determined which component of dairy products is responsible for effects such as increase in insulin resistance. In a separate study, the effects of low-fat dairy products showed to be insignificant after taking out calcium and vitamin D (Pittas AG., 2006). One explanation is that calcium and vitamin D can help in lowering the body fat and increase the fat oxidation (Zamel MB., 2004). Vitamin D has been recognized for its effects on stimulating insulin response through multiple mechanisms, including control of insulin receptors expression and stimulation of insulin release by pancreatic β-cells (Maestro B., 2000). This helps the glucose transport inside body (Rideout T., 2013). Additionally, there has been some concerns regarding rising of blood lipids following high consumption of dairy products. However the study by Rideout et al. suggest that low-fat dairy products can be included into a diet program without having any of mentioned adverse effects. Quiz to Test your Knowledge! adas.png|Questions adas2.png|Answers! Works Cited Beal, C., Louvet, P. and Corrieu, G. (1989) Influence of controlled pH and temperature on the growth and acidification of pure cultures of Streptococcus thermophilus 404 and Lactobacillus bulgaricus 398.Appl.Microbiol.Biotechnol. '32, '''148-154 Courtin, P. and Rul, F. (2004) Interactions between microorganisms in a simple ecosystem: yogurt bacteria as a study model. ''Le Lait. '84, '''125-134 Dairy Products - A Global Strategic Business Report (2012). Global Industry Analysts Inc. Diane, H. How to make homemade yogurt. Illustrated Bites. January, 17, 2011, http://illustratedbites. wordpress.com/2011/01/17/how-to-make-homemade-yogurt/(accessed October 28, 2013). Hamdan, I., Kunsman Jr, J. and Deanne, D. (1971) Acetaldehyde production by combined yogurt cultures. ''J.Dairy Sci. '54, '''1080-1082 Horiuchi, H., Inoue, N., Liu, E., Fukui, M., Sasaki, Y. and Sasaki, T. (2009) A method for manufacturing superior set yogurt under reduced oxygen conditions. ''J.Dairy Sci. '92, '''4112-4121 Lee, W. and Lucey, J. (2010) Formation and physical properties of yogurt. ''Asian-''Aust.J.Anim.Sci. 23, 1127-1136 Macdonald, M. (2013). Metabolism Slides. Retrieved from avenue.mcmaster.ca Ott, A., Hugi, A., Baumgartner, M. and Chaintreau, A. (2000) Sensory investigation of yogurt flavor perception: mutual influence of volatiles and acidity. ''J.Agric.Food Chem. '48, '''441-450 Pittas, A.G., Dawson-Hughes, B., Li, T., Van Dam, R.M., Willett, W.C., Manson, J.E. and Hu, F.B. (2006) Vitamin D and calcium intake in relation to type 2 diabetes in women. ''Diabetes Care. '29, '''650-656 Rideout, T.C., Marinangeli, C.P., Martin, H., Browne, R.W. and Rempel, C.B. (2013) Consumption of low-fat dairy foods for 6 months improves insulin resistance without adversely affecting lipids or bodyweight in healthy adults: a randomized free-living cross-over study. ''Nutrition journal. '12, '''56 Ruidavets, J., Bongard, V., Dallongeville, J., Arveiler, D., Ducimetière, P., Perret, B., Simon, C., Amouyel, P. and Ferrières, J. (2007) High consumptions of grain, fish, dairy products and combinations of these are associated with a low prevalence of metabolic syndrome. ''J.Epidemiol.Community Health. '61, '''810-817 Sidel, J.L. and Stone, H. (1993) The role of sensory evaluation in the food industry. ''Food Quality and ''Preference. 4, 65-73 Stone, H. and Sidel, J.L. (2004) Sensory evaluation practices. Academic press. Tong, X., Dong, J., Wu, Z., Li, W. and Qin, L. (2011) Dairy consumption and risk of type 2 diabetes mellitus: a meta-analysis of cohort studies. Eur.J.Clin.Nutr. 65, 1027-1031 Wild, S., Roglic, G., Green, A., Sicree, R. and King, H. (2004) Global prevalence of diabetes estimates for the year 2000 and projections for 2030. ''Diabetes Care. '27, '''1047-1053 Yale-New Haven Nutrition Advisor (2005) Understanding yogurt. online http://web.archive.org/web/20080529005611/http://www.ynhh.com/online/nutrition/advisor/yogurt.html Zemel, M.B. (2004) Role of calcium and dairy products in energy partitioning and weight management. ''Am.J.Clin.Nutr. '''79, '''907S-912S